1. Field of the Invention
The present invention relates to a laser transfer machining apparatus provided with a means for simultaneously generating a plurality of images to be transferred for laser machining, such as a hologram element.
2. Description of the Related Art
FIG. 1 is a schematic view of a known laser transfer machining apparatus, for example, an apparatus disclosed in Proceeding of SPIE, Vol.1377 p30-35. Shown in the figure are a laser oscillator 1, a deflecting mirror 2, a homogenizing optical system 3 for laser beam illuminating a mask, a mask 6, a transferring lens 7 for transferring a mask pattern image of the mask 6, and a target 8 to be machined which is an object to be machined.
In operation, the intensity distribution of a laser light emitted by the laser oscillator 1, e.g. an eximer laser is made uniform by the homogenizing optical system 3 for a laser beam illuminating a mask and then the uniform laser beam enters the mask 6. The laser light passing through the pattern to be transferred of the mask 6 is imaged as an transferred image on the target 8 by the transferring lens 7 and the illuminated part of the target is machined. The laser beam has been deflected by the deflecting mirror 2 before it is incident on the target 8.
Furthermore, as disclosed in Applied Optics Vol.13 No.2 p269-273, Japanese Patent Laid-open No.51-73698, Japanese Patent Laid-open No.54-102692, Japanese Patent Laid-open 57-81986, and Applied Optics Vol.30 No.25 p3604-3606, an optical system for machining with a hologram can be used in order to perform a laser machining operation as well as the laser transfer machining apparatus shown in FIG. 1. FIG. 2 is a schematic view of a conventional optical system using a hologram element which is called "Modulated Zone Plate", or a kind of hologram, which is disclosed in Applied Optics Vol.13 No.2 p269-273. Shown in FIG. 2 are a laser oscillator 1, a hologram element 5, a target 8 and an optical system 24 for expanding incident laser light.
In operation, a laser beam emitted by the laser oscillator 1 is expanded in size by the optical system 24 for expanding incident laser light and then is incident on the hologram element 5. A pattern to be machined is generated by the diffraction of the laser light incident on the hologram element 5 and is imaged on the target 8 to be machined. Thus, a few of the patterns are drilled simultaneously on the target. FIG. 3 is a schematic view of a known optical system for machining using a hologram disclosed in Japanese Patent Laid-open No.57-81986. A laser beam emitted by the laser oscillator 1 is expanded in size by the optical system 24 for expanding incident laser light and then is incident on the hologram element 5. The hologram element 5 is designed such that the laser beam passing through the hologram element has a predetermined pattern to be machined. The pattern is imaged on the surface to be machined of the target 8. Thus, the predetermined pattern is drilled on the target 8. The basic structure of the conventional optical system is the same as that of the system of FIG. 2.
Such a conventional laser transfer machining apparatus suffers from a drawback that most of a laser light incident on the mask enters an opaque part of the mask and therefore cannot pass through the mask, and this results in reducing the efficiency of utilization of light. For example, in a method of transfer machining using an eximer laser, in general, only a small part of the whole surface of a workpiece is machined. When the method is applied for a machining application to drill holes for conducting in a polyimide board in use for an electronic circuit, in general, about one hundred holes with diameters of about 100 .mu.m are drilled per square cm. In such a case, the ratio of the area of a part machined to the area of the whole surface of the board is 0.8% or less. When such a machining operation is performed in the conventional laser transfer machining apparatus shown in FIG. 1, most of energy of the incident laser light is scattered and absorbed by the mask and 0.8% or less of the beam power of the laser light emitted by the laser oscillator is utilized for the laser machining operation. Thus, a significant disadvantage of the prior laser transfer machining apparatus is that the efficiency of machining is very low.
When a hole drilling operation as mentioned above is performed using the laser transfer machining apparatus as shown in FIG. 1, there is an applicable method of scanning a laser light beam at many times, with the beam area on the workpiece being narrowed for the purpose of utilizing the laser light efficiently, such as a method of illuminating the workpiece with a light spot to drill holes one by one. However, since most of the region illuminated by the laser beam should transmit the laser light in order to improve the efficiency of utilization of light substantially, the area of the region illuminated at once should be very small. In the case of hole drilling, holes will be drilled one by one or two by two. Therefore, the whole region to be machined of a workpiece will be divided into many very small regions and the divided regions will be machined successively. Thus, in the case that this type of machining method is applied, the time required for machining is increased and hence the efficiency of machining becomes lower since the method wastes time for scanning a laser beam, followed by positioning the workpiece.
The machining optical system including the aforementioned conventional holograms was intended to eliminate the drawback of this type of transferring optical system. However, in the optical system for laser machining using the prior hologram as shown in FIG. 2, the hologram should be illuminated by a laser light with highly spatial and temporal coherence in order to re-create a precise pattern to be machined. The pattern reproduced by the hologram directly depends on the quality (or coherence) of the laser beam incident on the hologram; that is, the accuracy or the like of a machined pattern obtained by the method of machining is determined by the performance of the hologram and the quality of the incident laser light. In the case of a low spatially coherent light such as a light emitted by an eximer laser, in order to obtain a precise pattern to be machined the spatial coherence of the light has to be improved by using a spatial filter as described in reference books for optics, e.g. "Optical engineering", pp.250-252, by K. Iizuka. The spatial filter consists of a plate 21 having a pin-hole, the diameter of which is very small. As shown in FIG. 4, the plate is arranged such that the pin-hole is at the image focal point f of a lens 20 to eliminate a light component with disturbed wave fronts from a light incident on the pin-hole. In order to eliminate the light component with disturbed wave fronts effectively, the diameter of the pin-hole should be equal to the width of the main lobe of the incident laser light at the focal point of the lens. When a low spatially coherent laser light is incident on such a spatial filter, only a part of the laser light can pass through the spatial filter and hence the transmissibility of the filter is reduced sharply and a high efficiency of utilization of light cannot be obtained. Therefore, the conventional optical systems using such holograms are able to use only a highly coherent laser and are not able to use a relatively low coherent laser such as an eximer laser.
As mentioned above, the efficiency of machining provided by such a conventional laser transfer machining apparatus is low. In addition, it is impossible to reduce the time required for machining without reducing the total reliability of the machining apparatus because the laser oscillator must output a high power laser light at the sacrifice of the life time and stability thereof. Furthermore, laser transfer machining apparatuses using a conventional hologram suffer from a disadvantage that the apparatus must use a laser which is able to emit a highly and spatially coherent light. Therefore, when using a relatively low coherent laser such as an eximer laser, an optical system for improving coherence of incident light such as a spatial filter is needed and this results in reducing the efficiency of machining very much.